This invention relates to a process for making a refractory filter for liquid ferrous and non-ferrous metal with improved priming action.
Rigid ceramic filters support themselves by being thick enough to resist the bending forces exerted upon them by the pressure drop of the liquid metal flowing through the filter. They are also made thick so that they may act as depth filters. However, the resulting large thermal mass requires that the liquid metal be substantially superheated to a temperature well above its liquidus (the temperature above which no solids are stable).
Flexible, fibrous filters made from refractory filaments or yarn are difficult to handle because of their inherent flexibility. They support the pressure of the liquid metal by generating membrane forces, changing into three-dimensionally curved shapes. This requires that they be firmly clamped around the periphery. In order to be clamped successfully between the two halves (cope and drag) by the sand mold they must be rough-surfaced and rigid, and their warp and woof must be bonded together to prevent unraveling at the edges.
Particulate additives are often placed in the mold so as to act as lately as possible during the casting process (U.S. Pat. No. 3,703,922, November/1972, C. M. Dunks and J. L. McCaulay). This minimizes fading of the benefits of the additions during the time lag between application of the addition and freezing of the liquid metal. However, there may not be sufficient mixing of the inoculant with the incoming metal because the inoculant, located at the bottom of the pouring basin, is not approached very closely by all of the liquid metal, and because a metal skin freezes promptly upon contact with the mold walls, before the liquid metal has had a chance to mix by turbulent flow inside the runners and mold cavity.
Ease of priming is a desired attribute of a filter; priming is the initial penetration of the relatively cold filter by the hot liquid metal. It is essential that the first liquid metal striking the filter not lose so much superheat that it freezes against the filter, because if that happens no more liquid can pass through the filter, ruining the cast by preventing filling of the mold. Priming is improved by a filter with large openings and small depth, small thermal mass, and slow heat transfer or low heat conductivity in the filter material. The performance of the filter is better if it has smaller openings and greater depth (for deep-bed filters), however, and so it is essential that heat loss from the liquid metal to the filter be minimized by insulating the filter from the liquid metal. It may not be possible to modify the heat-flow characteristics of the filter material, and so a coating that can retard heat transfer is of great value in improving priming.
Soot deposited from a smoky flame has long been used as a mold coating (particularly for metal molds), because it prevents sticking between the mold and the casting, and it reduces thermal shock to the mold. A ceramic coating has been placed on a molybdenum metal screen to prevent reaction between the screen and the metal being filtered (U.S. Pat. No. 3,598,732, August/1971, D. W. Foster and W. G. Scholz). A resinous chill wash has been used to prevent sticking of the chill to the casting; the object of this invention (U.S. Pat. No. 2,816,336, December/1957, P. J. Neff) was to interpose a thin layer of evolving gas between the chill and the casting.
A mold wash has been patented (U.S. Pat. No. 3,115,414, December/1963, N. M. Lottridge, Jr. and D. G. McCullough), consisting of finely pulverized mica and wood flour in a liquid carrier, and which was intended to increase the fluidity of molten cast metals by retarding heat transfer from the liquid metal to the mold. This invention was limited to less than 50 percent carbonaceous solids (pitch plus wood flour) in the coating composition, because mica or vermiculite was to be the primary insulating component.
Pyrolytic graphite (U.S. Pat. No. 3,284,862, November/1956, W. H. Schweikert) has been applied to metal molds as a substitute for solid graphite or metal molds; it functions as an insulator because of the highly anisotropic heat conductivity of pyrolytic graphite, making the coating an insulator against the flow of heat perpendicularly into the mold surface.
Acenaphthylene (U.S. Pat. No. 3,692,550, September/1972, R. E. Melcher and R. W. Somers) has been used as a bulk additive to molding sand to prevent adherence (burn-on) of sand particles to a casting surface because upon exposure to the liquid metal it (a) forms a refractory, graphite-like coating on the exposed sand particles or (b) simply volatilizes and redeposits deeper within the sand without bonding the sand to the casting.